The two principal hypotheses to be tested are: RVEc vaccine will display an acceptable safety profile as determined by adverse event data and RVEc vaccine will elicit ELISA immunoglobulin G (IgG) titers greater than or equal to 1:500 and toxin-neutralizing antibody (TNA) anti-ricin toxin-neutralizing antibody titers greater than or equal to 1:50 in vaccine recipients.